AI Prompts for Cursor for Unit Testing
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Most developers write tests as an afterthought, and it shows. Tests that test implementation instead of behavior break every time the code is refactored. Tests that only cover the happy path miss the bugs that actually reach production. Cursor can write a good test suite from the function implementation, but only if you prompt it correctly: asking for behavior coverage not just line coverage, specifying edge cases explicitly, and asking it to use the patterns that the rest of your test suite follows. These prompts cover the full testing workflow from writing the first test to building a complete suite.
Stage 1
Plan the test coverage
Good tests start with knowing what to test. These prompts map out the behavior paths before writing any test code.
Map all the behaviors that need tests
Read this function and list every distinct behavior path that should be tested. Do not write test code yet, just list: the happy path, each error or failure case, edge cases involving null/undefined/empty values, boundary conditions, and any behavior that changes based on state or environment. I will review the list before we start writing tests. Function: [PASTE FUNCTION].
Identify the highest-risk untested paths
The following function is partially tested. Here are the existing tests: [PASTE TESTS]. Here is the function: [PASTE FUNCTION]. Identify: which behavior paths are not covered by the existing tests, which uncovered paths are highest risk (most likely to produce a bug or hardest to debug if they fail), and which tests would add the most value if written first. Give me a prioritized list.
Decide what not to test
For this module, help me decide what not to unit test. Review the code and identify: behaviors that are better tested at an integration or end-to-end level, framework behavior that does not need to be tested because the framework is already tested, and trivial logic where a test adds maintenance cost without meaningful coverage benefit. Code: [PASTE].
Design the test structure before writing
Before writing any test code, help me design the test structure for this function. Suggest: how to group the tests (by method, by scenario, or by input type), what to call the describe and test blocks so they read like documentation, and whether any test helpers or factories are needed to avoid repetitive setup. Function: [PASTE].
Identify what test data I need to prepare
To test this function thoroughly, I will need specific test data. Based on this function: [PASTE], list the test inputs I will need to create. For each test scenario, describe the input that triggers it, the expected output or behavior, and whether I can use a simple literal value or need a more complex fixture. Group by test scenario.
Stage 2
Write the tests
With a clear plan, these prompts write complete, well-structured tests for each coverage category.
Write a complete test suite for a function
Write a comprehensive test suite for this function using [TEST FRAMEWORK, e.g. Jest / Vitest / pytest / Go test]. Cover: the main happy path with at least two representative inputs, each documented error or failure case, null/undefined/empty edge cases, and any boundary conditions. Use descriptive test names that read like specifications. Do not test implementation details, only observable behavior. Function: [PASTE].
Write tests for error handling
Write tests that specifically verify the error handling in this function. For each error case: write a test that triggers that specific error, verify that the error is thrown or returned with the correct type and message, and check that the system is in the expected state after the error. If the function catches and re-throws errors, test that the re-thrown error contains the correct context. Function: [PASTE].
Write parameterized tests for multiple input variations
This function should produce the same type of output for many different inputs. Write parameterized tests using [FRAMEWORK] that cover [DESCRIBE THE INPUT RANGE OR VARIATIONS]. Use a test.each or equivalent structure so the test cases are easy to add to. Function: [PASTE]. Sample inputs and expected outputs: [LIST IF YOU HAVE THEM].
Write tests for async behavior
Write tests for this async function that cover: successful resolution with the correct value, rejection with the correct error, timing-dependent behavior if relevant, and whether concurrent calls behave correctly. Use [TEST FRAMEWORK] and show how to properly handle async assertions so tests do not pass incorrectly due to unhandled promise rejections. Function: [PASTE].
Write integration tests for a module
Write integration tests for [MODULE NAME] that test the interactions between its components rather than each function in isolation. These tests should: set up the relevant state or dependencies, call the module through its public API, and assert on the observable outcome rather than internal state. Use real dependencies where practical and mocks only where necessary. Module: [PASTE].
Stage 3
Improve existing tests
Tests that exist are not always tests that help. These prompts identify and fix weak tests.
Review my tests for common problems
Review this test suite and identify any tests that: test implementation details instead of behavior, are so tightly coupled to the current code that they would break on any refactor, test multiple things in one assertion (making failures hard to diagnose), have unclear names that do not describe what is being tested, or do not actually assert anything meaningful. For each problem found, suggest a fix. Tests: [PASTE].
Improve a flaky test
This test is flaky, it sometimes passes and sometimes fails without the code changing: [PASTE TEST]. Common causes of flaky tests are: reliance on timing, shared mutable state between tests, dependence on test execution order, non-deterministic data like dates or random numbers, and reliance on external services. Diagnose why this test is flaky and rewrite it to be deterministic.
Reduce test setup duplication
These tests have a lot of repeated setup code: [PASTE TESTS]. Refactor them to: extract shared setup into beforeEach or fixtures, create factory functions for common test objects, and reduce the noise in each individual test so the meaningful assertion is easy to find. The tests themselves should be more concise after refactoring.
Add missing assertions to an incomplete test
This test runs but is not asserting enough to be useful: [PASTE TEST]. Based on what the function does: [PASTE FUNCTION], add the missing assertions. The test should verify not just that the function runs without error but that it produces the correct output, handles state correctly, and calls any dependencies it should call.
Convert snapshot tests to behavior assertions
These snapshot tests are fragile and break whenever the output changes for any reason: [PASTE SNAPSHOT TESTS]. Rewrite them as explicit behavior assertions that check for specific properties or values rather than the full serialized output. The rewritten tests should only fail when behavior actually changes, not when cosmetic output changes.
Stage 4
Build testing habits and infrastructure
Good tests depend on good testing infrastructure. These prompts set up patterns that make testing easier to maintain.
Create test factories for common objects
I am repeatedly creating the same types of test objects in multiple test files. Create factory functions for these common test objects so tests can create them with defaults and override only what they need. Objects to create factories for: [LIST]. Use the builder or factory pattern. Each factory should: have sensible defaults, accept overrides for any property, and be easy to use inline in a test.
Set up mocks for common dependencies
I need to mock these dependencies consistently across my test suite: [LIST DEPENDENCIES, e.g. database client, HTTP client, logger]. Create reusable mock factories for each one that: set up the mock with sensible defaults, can be easily configured for specific test scenarios, and reset properly between tests. Use [TEST FRAMEWORK] mocking utilities.
Write a testing guide for this codebase
Based on the testing patterns in this codebase, write a short testing guide (under 500 words) that covers: which test framework and assertion library we use, how to run tests locally, the naming conventions we use for test files and test cases, when to use unit tests versus integration tests versus mocks, and where to find examples of well-written tests in the codebase. Existing tests for reference: [PASTE A FEW EXAMPLES].
Configure test coverage thresholds
Help me configure coverage thresholds for this project using [TEST FRAMEWORK]. I want to: enforce minimum coverage on branches and lines, exclude [FILES OR PATTERNS TO EXCLUDE, e.g. config files, type definitions, index files] from coverage requirements, and get failing CI when coverage drops below the threshold. Show me the configuration to add to [CONFIG FILE, e.g. jest.config.js, vitest.config.ts].
Add a test for a bug before fixing it
There is a bug in this function: [DESCRIBE THE BUG]. Before fixing it, write a failing test that reproduces the bug. The test should: clearly demonstrate the incorrect behavior, fail with the current code, and pass once the bug is fixed. Having this test ensures the bug cannot be reintroduced. Function: [PASTE].
Frequently asked questions
How does Cursor know what tests to write without seeing the tests that already exist?+
It does not, so always paste the existing test file alongside the function when using the test-writing prompts. Cursor then matches the patterns, naming conventions, and mocking approach already in use rather than introducing a new style. If you do not have existing tests, describe your testing framework and any conventions you want to follow.
Should I write tests before or after the code?+
The answer depends on what you are building. For a well-understood behavior, writing tests first (TDD) is faster because it forces you to define the interface before the implementation. For exploratory code where you are still figuring out the design, write the code first, then write tests before refactoring. Cursor is useful in both workflows.
What is the difference between a unit test and an integration test, and which should I write first?+
A unit test tests a single function or module in isolation, with dependencies mocked. An integration test tests multiple components working together. Write unit tests first for the core logic because they are faster to run and easier to debug. Add integration tests for the workflows that matter most to users, focusing on the paths most likely to have interaction bugs.
How do I test a function that has side effects or depends on external state?+
Isolate the side effects behind an interface you can swap in tests. If the function writes to a database, pass the database client as a parameter and mock it in tests. If it reads from an environment variable, wrap that in a function you can override. The dependency injection principle applies to test setup too. The mock setup prompt in Stage 4 covers the most common dependency types.
What is the biggest mistake in Cursor-generated tests?+
Testing implementation instead of behavior. Cursor sometimes generates tests that call private methods, assert on internal state, or check exact call counts on mocks in ways that break whenever the implementation changes even if the behavior is the same. The review prompt in Stage 3 specifically flags tests that are too tightly coupled to the implementation.
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